Powdery mildew

Damage
The white, fluffy, later grey-tan colonies of powdery mildew are easily detected on leaves stems and ears, although the upper surface of leaves are most commonly infected. Before the development of white patches producing masses of conidia and causing the powdery appearance chlorotic flecks on the plant tissue appear as initial symptoms. Cleistothecia may be formed occurring late in the season and found embedded in the mildew colonies as dark-coloured dots. Under favourable conditions, sporulation starts within 7-8 days after infection. The disease affects flowering, fruiting, seedling and vegetative growing stages of the plants. Conidia are dispersed within the crop by the wind. Epidemics will tend to occur during conditions of alternating wet and dry weather.

Cause
Powdery mildew is caused by Blumeria graminis, an ascomycete belonging to the Erysiphales. It is an obligately parasite specific to wheat and overwinters mainly as mycelium on autumn-sown cereals. Although cleistothecia releasing ascospores produced during the late summer are resistant to cold and drying out, they appear to be of secondary importance in overwintering and as a source of inoculum in the spring. With rising temperatures in spring, conidia are produced rapidly on dormant mycelium beginning with growth. Conidia usually germinate over a wide range of temperatures from about 5 ° - 30 °C, although 15 ° - 20 °C is probably optimal, together with a relative humidity of about 95 %. Free water inhibits conidial germination.